One of the most common power amplifier configurations is the "H-bridge" as shown in FIG. 1. This configuration typically uses one power supply and four power switches (i.e. power transistors) to produce the desired current flow through a load. The load is the device to which power is delivered. It can be resistive, capacitive, or inductive. The advantage of the H-bridge configuration is that it uses only one power supply. The current to the load is controlled by means of the four power switches. During one half cycle, current flows from switch A through the load to switch D. During the other half cycle, current flows from switch B through the load to switch C.
There are, however, several disadvantages to using an H-bridge. The H-bridge requires four power switching devices for each load. Since power switching devices are usually the most expensive components of an amplifier, the cost of this configuration increases if several loads are driven by the same power supply. There is also a size limitation. The entire power amplifier must be large enough to accommodate four power switching devices. One other disadvantage is that during any half cycle power is always being dissipated in two power switching devices, either A and D or B and C.
An alternative power amplifier configuration is the "totem pole" or T-configuration as shown in FIG. 2. An advantage of this configuration is that it only requires two power switching devices and, therefore, can be made less expensive and more compact. Another advantage is that during any half cycle power is dissipated in only one power switch, thus minimizing power consumption.
The T-configuration, however, requires two power supplies, one positive and one negative. This is usually more costly than one power supply, but in a configuration where multiple loads are driven or where multiple amplifiers are operated from the same power supplies, the cost savings in power switching devices will offset the increased cost of two power supplies.
In many applications, the T-configuration is used to supply power to an inductive load. With an inductive load, however, there is a serious problem with using the T-configuration because the power supply voltages, if left unregulated, could reach destructive levels. There are devices which protect the power switching devices from excessive voltage levels, such as shown in U.S. Pat. Nos. 4,318,162 and 4,378,580; however, these do not protect the power supplies.
To illustrate the problem, consider the situation shown in FIG. 3 where an amplifier is supplying current to an inductive load. Initially, when power switch A turns on, current will flow into load L in the direction indicated by I. This current is being supplied by the positive power supply.
When the current in the load reaches a certain value, switch A is turned off. The turn-off value is typically detected by a current sensing means, usually a resistor, which is in series with load L. Since the load is inductive, the current therein cannot change instantaneously and must continue to flow in the same direction through the load until it decays. To enable this to happen, the current must flow through the diode around switch B as shown in FIG. 4 since it cannot flow through either switch A or switch B because both are turned off.
The current I that flows through the diode around switch B is fed back into the negative power supply. Typically, the negative power supply will be as shown in FIG. 5. The current I that is fed back into the negative power supply will flow into capacitor C in the direction shown in FIG. 5 and will charge the capacitor at a rate of I/C causing the voltage across capacitor C to increase. If nothing is done to correct this change in voltage, the actual magnitude of the voltage across capacitor C will, in time, reach potentially destructive levels, both to capacitor C and to power switches A and B.
During the switching action indicated by FIGS. 3 and 4, current drawn from the positive power supply is fed into the negative power supply since it was the positive power supply which initially generated the current I flowing in load L. Alternatively, if negative current was being fed into load L by power switch B and then switch B was turned off, current drawn from the negative power supply would be fed into the positive power supply resulting in a similar destructive change in voltage or build-up as has been described for the negative power supply.
Since power is supplied to the load, the average current in load L cannot be zero. As a result, the switching action between the power supplies does not cancel the power supply build-up. The voltage of each supply continues to increase with each cycle. There is a need, therefore, for a device which will correct this power supply build-up enabling one to construct a safe and reliable single or multiple power amplifier system in a cost effective manner using a minimum number of power switches.